Gyrotron ESR in CsFeCl3 under High Magnetic Fields

More than ten years ago, an anomalous magnetization was observed in CsFeCl3 under the magnetic field exceeding 33 T. Usually, the spin state of Fe2+ in CsFeCl3 is treated as the fictitious spin S=1 with the singlet ground state due to the crystallographic single ion anisotropy. The magnetization saturates at the magnetic field of about 10 T applied parallel to the crystal c-axis. The anomalous magnetization can not be expected within the framework of the fictitious spin S=1. In order to study the relation between the anomalous magnetization and the excited spin multiplet J=2, an ESR experiment was performed under magnetic fields up to 40 T by use of a pulse magnet. The submillimeter wave was supplied by Gyrotron FU IV at Research Center of Far Infrared Region, Fukui University. The ESR absorption lines were observed corresponding to the anomalous magnetization jump